Concentrations of Nitric Oxide in Laminar Counterflow Methane/Air Diffusion Flames

Abstract

Results of nearly 20 different measurements of profiles of concentrations of oxides of nitrogen in steady counterflow methane-air flames are studied and compared with predictions of four different chemical-kinetic mechanisms. The conditions include air-side strain rates between about 10 and 200 s - 1 , pressures between 1 and 15 atm, air temperatures between about 300 and 560 K, partial premixing of air on the fuel side down to an equivalence ratio of 1.5, and dilution of the fuel with nitrogen down to a fuel mole fraction of 0.25, all for a fuel temperature of about 300 K. Although most of the experimental results were taken from the literature, two new measurements were made, employing a newly devised NO x -scrubber technique to correct chemiluminescent NO x -analyzer results for interference from other species, principally hydrocarbons on the fuel side containing two or more carbon atoms. The results show that differences between measured and predicted NO x concentrations tend to be smaller on the fuel-lean side of the flame than they are on the fuel-rich side, where typical discrepancies reach values on the order of 50%. Whereas the Milan mechanism, the San Diego mechanism, and the Gas Research Institute (GRI) mechanism version 2.11 usually produce reasonable agreement with the experimental data, the newer GRI mechanism version 3.0 tends to overpredict NO x under most of the conditions investigated. Because the prompt mechanism is the principal source of NO x in these flames, this might be associated in some way with the tendency of the GRI mechanisms to overpredict acetylene concentrations by about a factor of two, an overprediction that is demonstrated to occur by comparisons with additional experimental data on concentration profiles of intermediate hydrocarbons.